J. Pleim

Sensitivity of continental United States atmospheric budgets of oxidized and reduced nitrogen to dry deposition parametrizations

Reactive nitrogen (Nr) is removed by surface fluxes (air–surface exchange) and wet deposition. The chemistry and physics of the atmosphere result in a complicated system in which competing chemical sources and sinks exist and impact that removal. Therefore, uncertainties are best examined with complete regional chemical transport models that simulate these feedbacks. We analysed several uncertainties in regional air quality model resistance analogue representations of air–surface exchange for unidirectional and bi-directional fluxes and their effect on the continental Nr budget. Model sensitivity tests of key parameters in dry deposition formulations showed that uncertainty estimates of continental total nitrogen deposition are surprisingly small, 5 per cent or less, owing to feedbacks in the chemistry and rebalancing among removal pathways. The largest uncertainties (5%) occur with the change from a unidirectional to a bi-directional NH3 formulation followed by uncertainties in bi-directional compensation points (1–4%) and unidirectional aerodynamic resistance (2%). Uncertainties have a greater effect at the local scale. Between unidirectional and bi-directional formulations, single grid cell changes can be up to 50 per cent, whereas 84 per cent of the cells have changes less than 30 per cent. For uncertainties within either formulation, single grid cell change can be up to 20 per cent, but for 90 per cent of the cells changes are less than 10 per cent.

Mitigation of severe urban haze pollution by a precision air pollution control approach

Severe and persistent haze pollution involving fine particulate matter (PM2.5) concentrations reaching unprecedentedly high levels across many cities in China poses a serious threat to human health. Although mandatory temporary cessation of most urban and surrounding emission sources is an effective, but costly, short-term measure to abate air pollution, development of long-term crisis response measures remains a challenge, especially for curbing severe urban haze events on a regular basis. Here we introduce and evaluate a novel precision air pollution control approach (PAPCA) to mitigate severe urban haze events. The approach involves combining predictions of high PM2.5 concentrations, with a hybrid trajectory-receptor model and a comprehensive 3-D atmospheric model, to pinpoint the origins of emissions leading to such events and to optimize emission controls. Results of the PAPCA application to five severe haze episodes in major urban areas in China suggest that this strategy has the potential to significantly mitigate severe urban haze by decreasing PM2.5 peak concentrations by more than 60% from above 300 μg m−3 to below 100 μg m−3, while requiring ~30% to 70% less emission controls as compared to complete emission reductions. The PAPCA strategy has the potential to tackle effectively severe urban haze pollution events with economic efficiency.

Ammonia Emissions in the US: Assessing the Role of Bi-directional Ammonia Transport Using the Community Multi-scale Air Quality (CMAQ) Model

A pilot study assessing bi-directional ammonia (NH3) transport using the Community Multi-scale Air Quality (CMAQ) Model for the Eastern United States (US) is underway. The study develops and tests bi-directional flux algorithms, explores methods of providing agricultural fertilizer information into CMAQ, and clarifies possible NH3, and overall one-atmosphere, chemical budget changes with the full implementation of the bi-directional flux option planned for the 2011 CMAQ release. One focus area is the adjustment of the current CMAQ bi-directional flux module to include a dynamic soil emission potential component. The soil emission potential (Γg) is calculated offline using commercial fertilizer application survey data from the National Nutrient Loss & Soil Carbon (NNLSC) Database and is then input to CMAQ for computation of the NH3 air-soil compensation point and subsequent NH3 flux. A full 2002 year model run over the standard CMAQ (v. 4.7) Eastern US domain incorporating the revised bi-directional flux module is planned.